The reason for screening out patients with naturally large pupils is that the extra light they let in can disrupt vision in the hypersensitive postsurgical eye.
Under normal circumstances, a person’s pupils adjust in size to let into the eye the proper amount of light under the available light conditions. If one is outside on a sunny day, one’s pupils grow tiny to prevent too much light from entering the eye. But if one is in a dark room, one’s pupils grow very large, expanding to retrieve as much light as possible. Everyone experiences the pupil-expansion process during the few seconds it takes for one’s eyes to adjust to the dark.
With laser eye surgery, small amounts of tissue are removed from the cornea in the center of the eye, sculpting the area so that light entering the eye will be focused correctly on the retina (the light-sensitive surface at the back of the eyeball). But under circumstances in which a person’s pupils are naturally large, the light entering the eye through the disturbed tissue area might produce the sight of a starburst pattern around lights at night. This makes driving difficult and blindingly dangerous, especially when driving into headlights.
LASIK on large pupils could also result in the sight of halos around lights, especially at night – which could again interfere with night driving.